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An Appendix filed herewith on two identical compact discs (Copy 1 and Copy 2) each containing one computer program listing constitutes a part of the specification pursuant to 37 CFR xc2xa7xc2xa71.77 and 1.96. Each compact disc contains one computer program listing named xe2x80x9cComputerProgramListingxe2x80x9d created on the compact discs on Jun. 3, 2002 of file size 15,956 bytes (16,384 bytes on disk) on each compact disc. The entirety of the computer program listing contained in the compact disc is incorporated by reference herein.
1. Field of the Invention
The present invention relates to lithography and to electron (or other energy) beam columns and more specifically to a structure and method for determining shape codes of variable shaped beams.
2. Description of the Related Art
It is well known in the field of lithography (pattern generation) that it is desirable to increase the throughput of pattern generation systems. Two main applications for such pattern generation systems are making masks for use in semiconductor fabrication by electron beam lithography and electron beam direct writing of patterns onto wafers to form semiconductor devices.
Lithography systems generate or expose patterns by controlling the flow of energy (the beam) from a source to a substrate coated with a layer sensitive to that form of energy. Pattern exposure is controlled and broken into discrete units commonly referred to as flashes, wherein a flash is that portion of the pattern exposed during one cycle of an exposure sequence. Flashes are produced by allowing energy from the source, for example light, electron or other particle beams, to reach the coated substrate within selected pattern areas. The details of flash composition, dose and exposure sequence used to produce a pattern, and hence the control of the lithographic system, make up what is known as a writing strategy.
A traditional raster scan writing strategy employs a uniform periodic raster scan, somewhat similar to television raster scanning. A mechanical stage moves a substrate, for example placed on a table, uniformly in a direction orthogonal to the direction of the uniform scan of an energy beam. In this manner a pattern is composed on a regular grid with a regular scan trajectory resulting from the orthogonal movement of the stage and beam. When the beam is positioned over a grid site requiring exposure, the beam is unblanked and the underlying site exposed. Only the amount of dose, or energy, at each site is varied as required. Hence, exposure data can be organized in a time sequence corresponding to the regular scan trajectory, and only the dose for each site need be specified. The distinguishing characteristics of a traditional raster scan writing strategy are a small round beam exposing one site at a time, a periodic scan moving sequentially to each site of a grid and a rasterized representation of data corresponding to the required dose for each site or xe2x80x9cpixelxe2x80x9d of the grid.
On the other hand, in a typical vector scan writing strategy, the beam is positioned only over those sites that require exposure and then unblanked to expose the site. Positioning is accomplished by a combination of stage and beam movement in what is often referred to as a semi-random scan. Thus, data must be provided that includes both the dose and position of each flash or site exposed. Frequently vector scan strategies use a variable shaped beam, that is a beam capable of having a different size and/or shape for each flash. The pattern is then composed from these variable shapes. A shaped beam is capable of exposing multiple pixel sites simultaneously instead of one pixel site at a time as in a raster scan writing strategy. Where a variable shaped beam is used, the data must additionally include the location, size and shape for each flash. Thus the distinguishing characteristics of traditional vector scan writing strategies are a variable shaped and sized beam exposing multiple pixel sites in a single flash, a semi-random scan encompassing only those portions of a pattern to be exposed, and a vectorized representation of data including the location, size, shape and dose of each flash.
Both vector and raster scan writing strategies have advantages and disadvantages. Vector scan strategies can offer fine pattern definition. However, vector scan flash rates are typically slower than raster scan strategies due to settling time required between the relatively large beam deflections of the semi-random scan trajectory. For patterns with exposed portions that are finely detailed, vector scan strategies are relatively slower due to delays in settling of the electron beam shaping components which are capable of shaping the beam over a wide range of dimensions. Also, current density (current per unit area) is generally lower in vector scan strategies due to the need for the electron source to be capable of covering larger areas simultaneously, again leading to lower throughput. A drawback of raster scan writing processes is a relatively coarse pattern definition.
Thus it is desirable to develop an improved writing strategy that combines the advantages of a vector scan strategy, namely, fine pattern definition, with those of a raster scan strategy, namely, increased speed, to increase the throughput of pattern generation systems.
An embodiment of the present invention provides a flash converter that determines shape data that specifies a flash field among pixels, the pixels being represented as gray level values that represent a proportion of each pixel that overlaps with a pattern, the flash converter including: a reformatter which constructs a matrix of a quadrant and surrounding pixels of the flash field, where the reformatter modifies the matrix so that N intermediate shapes correspond to an exposed region of the quadrant are provided; and a shape code determinator coupled to receive the modified matrix and which determines an intermediate shape data that specifies the quadrant, where the shape code determinator performs a reverse modification on the intermediate shape data and outputs shape data that specifies the flash field. In one embodiment, N is 3.
An embodiment of the present invention provides a method for determining shape data that specifies a flash field among pixels, the pixels being represented as gray level values that represent a proportion of the pixel that overlaps with a pattern, including the acts of: constructing a matrix of a quadrant and surrounding pixels; modifying the matrix so that N intermediate shapes corresponding to an exposed region of the quadrant are provided; determining an intermediate shape data that specifies the quadrant; and performing a reverse modification on the intermediate shape data to determine the shape data that specifies the flash field. In one embodiment; N is 3.
The present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.